Clarifying Subdomain Access, Design Sharing, and Billing in Organizations

I’d like to clarify a few things:

Imagine we have bigdomain.com, and there’s another group using marketing.bigdomain.com that already has an Organization account. Now, I want to create my Organization account using the domain bigdomain.com, including everything that is *.bigdomain.com.

1. Can the existing marketing.bigdomain.com account join our teams, and will they have access to our bigdomain.com organization’s libraries?

2. Can bigdomain.com share a design system with someone in marketing.bigdomain.com without making them an “editor” in one of our teams?

3. Who covers the costs? If they already pay through their organization marketing.bigdomain.com, will bigdomain.com also has to pay?

4. What about a “Guest” with a Professional account? Do we need to pay for them to join one of our teams in bigdomain.com?

I appreciate your guidance. Figma’s support has been slow to respond, despite my multiple emails.